A number of electronic devices using a group III nitride film, such as GaN, InGaN, InN or InAlN, on a substrate have been manufactured in recent years as electronic devices, such as blue light emitting diodes.
The device substrate is often formed of a sapphire film. The use of a single crystallin silicon substrate, which can be supplied at low cost and in large quantities, has been studied. The silicon substrate can also advantageously have a high thermal conductivity and therefore withstand high power operation compared to the sapphire substrate has.
When the group III nitride film is formed on a silicon substrate, it is necessary to form a buffer layer on the substrate in order to reduce lattice defects. The present inventors have previously proposed that a double buffer layer of Si3N4 and AlN are formed between the silicon substrate and the group III nitride film (see Patent Document 1).
A method for forming the double buffer layer between the Si substrate and the group III nitride film will be briefly explained with reference to FIGS. 17 to 19. FIG. 17 is a diagram schematically showing a configuration of an MBE (Molecular Beam Epitaxy) growth equipment that is used to form the buffer layers on the silicon substrate.
The MBE growth equipment 1 includes an RF (Radio Frequency) excitation cell 4 and a metal molecular beam cell 5 that are provided in a vacuum chamber (growth camber) 3, and an RF matching box 6, an RF power supply 7 and a personal computer (hereinafter referred to as a “PC”) 8 that are provided outside the vacuum chamber 3. A counter electrode body 11 of an atomic flux measurement device 10 is provided in the vacuum chamber 3 in the vicinity of a substrate holder 31, while the main body of the atomic flux measurement device 10 is provided outside the vacuum chamber 3 and is connected to the PC 8 via a cable.
The vacuum chamber 3 is maintained at high vacuum (10−4 to 10−8 Pa residual pressure) using a turbo molecular pump (not shown). A silicon substrate 2 that has already been subjected to the cleaning treatment is fixed to the substrate holder 31 and is heated to a predetermined temperature using a heater (not shown).
Although not shown, a shroud is provided on a sidewall surface of the vacuum chamber 3. The inside of the shroud is filled with liquid nitrogen. In the vacuum chamber 3, gas molecules are adsorbed by the wall surface when the gas molecules strike the sidewall, whereby a high degree of vacuum can be maintained.
The RF excitation cell 4 and the metal molecular beam cell 5 are provided in the wall of the vacuum chamber 3 to emit nitrogen atoms and metal molecules (e.g., Ga) toward the silicon substrate 2 held by the substrate holder 31. FIG. 18 shows a detailed structure of the RE excitation cell 4 that generates nitrogen gas plasma. Nitrogen gas supplied via a gas port 45 from a nitrogen gas cylinder (not shown) is supplied into a discharge chamber 42 of a hollow crucible 41. The amount of the nitrogen gas supplied is adjusted by a flow rate controller 46. An excitation coil 43 that also serves as a water cooling pipe is coaxially wound around the outer circumference of the crucible 41. By circulating cooling water W, the crucible 41 and the RF excitation cell 4 are cooled.
When high-frequency power is supplied to the excitation coil 43 from the RF power supply 7 via a terminal 63 of the RF matching box 6, the nitrogen gas in the discharge chamber 42 is excited into a plasma state so that a supersonic jet of an active species F of nitrogen is emitted through an orifice 44 provided in an output portion.
The metal molecular beam cell 5 melts a solid metal material (e.g., Ga) put in the crucible using a heater, and emits evaporated atoms toward the substrate 2 by opening and closing a shutter 9 attached to the output portion. Although the single metal molecular beam cell 5 is shown in FIG. 17, the vacuum chamber 3 typically includes a plurality of the metal molecular beam cells 5, the number of which depends on the number of metal molecules used.
The RF matching box 6 is provided to perform impedance matching between the RF power supply 7 and the plasma in the discharge chamber 42 so that the high-frequency power applied from the RF power supply 7 to the RF excitation cell 4 is smoothly supplied to the discharge chamber 42. The RF matching box 6 includes an automatic reactance adjustment circuit 61 and a variable reactance circuit 62.
In the above MBE growth equipment 1, the RF excitation cell 4 can be operated in two discharge modes. The first discharge mode is called an “HB discharge mode” in which a relatively high degree of high-frequency power (e.g., 500 W) is applied to the excitation coil 43 to excite nitrogen gas in the discharge chamber 42, whereby nitrogen plasma having a high brightness is obtained. In the HB discharge mode, as shown in spectrum line diagram of FIG. 19, emission of a flux (N+N*) of dissociated nitrogen atoms was observed including ground-state atoms N and excited atoms N* that are generated by dissociation of nitrogen molecules N2, and excited nitrogen molecules N2*, nitrogen molecule ions N2+ and electrons.
A second discharge mode is called an “LB discharge mode” in which a relatively low degree of high-frequency power (e.g., 120 W) is applied to the excitation coil 43 to excite nitrogen gas in the discharge chamber 42, whereby nitrogen plasma having a low brightness is obtained. In the LB discharge mode, no flux (N+N*) of dissociated nitrogen atoms was contained in plasma emitted from the RF excitation cell 4, and emission of excited nitrogen molecules N2*, nitrogen molecule ions N2+ and electrons was observed.
The present inventors have extensively studied characteristics of the plasma generated in the HB discharge mode by conducting a variety of experiments. As a result, the present inventors have found that the excited atoms N* and the ground-state, atoms N contained in the plasma in the HB discharge mode are so-called metastable atoms, which have a thermodynamically relatively long life (of the order of milliseconds). On the other hand, the molecular ions N2+ and the electrons have the property that they quickly disappear due to recombination in the vacuum chamber. The present inventors also have found that the excited molecules N2*, the excited atoms N* and the ground-state atoms N contained in the plasma in the HB discharge mode are readily attached to a solid-phase interface, such as the substrate surface and the metal plate surface.
When a crystalline layer of GaN or AlGaN is grown on the silicon substrate, the substrate 2 is preferably directly irradiated with the high-energy excited atoms N* and ground-state atoms N emitted from the RF excitation cell 4. Such an irradiation technique is hereinafter referred to as “direct irradiation.” In contrast to this, when a buffer layer is formed on the silicon substrate, the substrate is preferably indirectly irradiated with an appropriate amount of low-energy excited atoms N* and ground-state atoms N. Therefore, when a buffer layer is formed, as shown in FIG. 17, the HB-discharge-mode plasma emitted from the RF excitation cell 4 is caused to strike and rebound off a reflection plate 32 provided in the vacuum chamber 3 and, in addition, the shutter or the shroud in the RF excitation cell 4, at least once, so that the energy is reduced, before striking the surface of the substrate 2. Such an irradiation technique is hereinafter referred to as an “indirect irradiation.”
Next, the step of forming the double buffer layer of Si3N4 and AlN between the silicon substrate and the group III nitride film using the above MBE growth equipment 1 will be briefly described. A treatment for cleaning the substrate surface is performed before the step of forming the Si3N4 buffer layer on the silicon substrate 2. The treatment is well known and therefore will not be described.
(1) The silicon substrate 2 that has been subjected to the cleaning treatment is fixed to the substrate holder 31 in the vacuum chamber 3, and is heated to a predetermined temperature using the heater.
(2) High-frequency power of, for example, 500 W having a frequency of 13.56 MHz is applied to the excitation coil 43 of the RF excitation cell 4 so that discharge occurs in the nitrogen gas in the HB discharge mode. The substrate is indirectly irradiated with a dissociated nitrogen atomic flux generated in the HB discharge mode, whereby a β-Si3N4 monocrystalline film is epitaxially grown by surface/interface reaction.
(3) The Si3N4 monocrystalline film is irradiated with an Al atomic flux corresponding to several atomic layers using an Al molecular beam cell, whereby an AlN monocrystalline film is epitaxially grown due to surface/interface reaction.
(4) The AlN monocrystalline film is directly irradiated with a dissociated nitrogen atomic flux and an excited nitrogen molecule flux that are generated in the HB discharge mode, and is also irradiated with an Al atomic flux using an Al molecular beam cell, whereby an AlN epitaxial layer is formed.
If a crystal of GaN or AlGaN is grown on the silicon substrate on which the double buffer layer have been formed by the above steps, a film having less lattice defects can be formed.
Incidentally, in order to control the growth operation of the MBE growth equipment 1 employing the RF excitation cell 4, it is necessary to monitor the amount of dissociated nitrogen atomic flux that strike the surface of the substrate 2. Conventionally, the amount of dissociated nitrogen atomic flux is measured using the Langmuir probe technique. However, the Langmuir probe technique is designed to measure a current flowing through a metal probe based on charged particles. As described above, particles (i.e., atoms and excited molecules) emitted from the RF excitation cell 4 are electrically neutral. Therefore, the amount of dissociated nitrogen atomic flux may not be correctly measured by the Langmuir probe technique.
The present inventors have previously developed an device for measuring the amount of dissociated nitrogen atomic flux (see Patent Document 2). This measurement device makes use of the phenomenon that when electrically neutral dissociated nitrogen atoms are attached to a probe electrode having a negative potential, the atoms emit electrons due to self ionization, whereby a current (hereinafter referred to as an “atomic current”) flows. The value of the atomic current flowing through the probe electrode varies depending on the amount of the atomic flux in an atmosphere in which the probe electrode is placed. Therefore, the amount of the atomic flux can be determined by measuring the value of the current.